The loss of material from stock piles or railcars represents both an economic loss and an environmental concern. For example, the loss of coal from an uncovered railcar during transport has been measured at about 5%, depending on the substrate and distance transported (Booth et al., U.S. Pat. No. 2,854,347). In addition, weathering of unprotected material can significantly degrade the value of that material as cited, for example, by Wattles (Wattles, U.S. Pat. No. 2,204,781). Finally, the material that is eroded from a railcar or stock pile does not simply disappear. It is re-deposited downwind of its origin, frequently to the dismay of property owners and the detriment of wildlife.
The re-deposition of material can have serious consequences. The Surface Transportation Board (STB) determined that coal dust coming off of open railcars and fouling of railroad ballast is a serious problem. The STB determined that the railroads had the right to require shippers to address the problem (STB docket number FD 35305). Coal dust was also shown in Tutumluer's paper (E. Tutumluer, W. Dombrow, H. Huang; “Laboratory Characterization of Coal Dust Fouled Ballast Behavior” AREMA 2008 Annual Conference & Exposition Sep. 21-24, 2008, Salt Lake City, Utah) to be capable of destabilizing railroad tracks after a heavy rain event, increasing the possibility of a derailment and the economic consequences attendant upon such an event.
Two methods of reducing the loss of material from stockpiles and railcars have been successfully employed in the past: treating the entire mass of material with a formula that reduces the material's inherent tendency to generate dust, or applying an encrusting agent to the exposed surface of the mass of material and forming a barrier against loss. The present invention is directed to an application of the latter technology.
There has been any number of methods put forth over many decades to address this. Wattles taught the application of molten paraffin wax to stock piles and railcars as early as 1939 (Wattles, U.S. Pat. No. 2,204,781). As natural and man-made latexes became commonly available in the early 1950's, they were also found to be effective in stemming the loss of material from stock piles and railcars (Booth et al., U.S. Pat. No. 2,854,347). At roughly the same time, vinyl addition polymers, especially water-soluble polyacrylates, polymethacrylates, polyvinyl acetate, and polyacrylamides, was similarly pressed into service as coatings for stock piles and rail cars (Booth et al., U.S. Pat. No. 2,894,851). Drying oil pile surface stabilizers, with or without catalysts were claimed in the early 1970's (Nimerick, et al., U.S. Pat. No. 3,708,319). At about the same time, either a polyacrylamide slurry or a dry polyacrylamide powder was shown to be effective in preventing wind erosion from stock piles and dry tailings ponds (Stout et. al., U.S. Pat. No. 3,677,014). Other examples of single compositions include the use of calcium sulfate (Mueller et al., U.S. Pat. No. 4,269,721), high molecular weight polyethyleneglycol (Burns et al., U.S. Pat. No. 4,316,811), or water-insoluble methacrylate copolymers (Kirwin, U.S. Pat. No. 4,594,268). The formulations also began to become more sophisticated in the late 1970's and 1980's, with combinations designed to address deficiencies in single compositions. Examples of this include: a silicone added to a latex (Nimerick, U.S. Pat. No. 4,087,572), an organic binder such as wax, tar, or asphalt combined with an organic filler such as coal (Kromrey, U.S. Pat. No. 4,214,875), coal tar emulsion combined with a wetting agent (Shaw et al., U.S. Pat. No. 4,264,333), water-soluble cellulose ethers with surfactants (Callahan, et. al. U.S. Pat. No. 4,369,121), an emulsified blend of coal tar pitch in an aromatic solvent and in which cellulose ethers were used as thickeners to aid in the stabilization of the emulsion (Kremer, U.S. Pat. No. 4,960,532), insoluble cellulose fibers applied either to the bulk of the dusting material or as a sealer over the surface of a storage pile and with or without an additional polymeric binder (Kestner, U.S. Pat. No. 4,836,945), a combination of emulsified anionic and water-soluble cationic polymers (Field, et al., U.S. Pat. No. 4,981,398). More recently compositions using cement (Walker, U.S. Pat. No. 5,530,596 and Johnson, U.S. Pat. No. 6,409,818), sugar (Cole, U.S. Pat. No. 5,595,782; Bytnar et al., U.S. Pat. No. 7,157,021; and Wynne et al., U.S. Pat. No. 7,854,857), gelatanized starch (Wolff, U.S. Pat. No. 7,976,724), and mixtures of guar gum (Marsden, et. al., U.S. Published Patent Application No. 20100301266) have all been proposed to abate material loss from stock piles and railcars.
From the above references, it is apparent that there are two critical parameters for a successful pile sealing formula: strength and thickness. Strength is essential as it relates to the basic mechanism by which a crust forms. A crust forms in most instances when a composition binds the loose particles of the bulk material at the surface into a cohesive whole. The stronger the bond between the bulk material particles, the stronger the crust and the less likely the crust will suffer from a mechanical failure that would expose unbound material which would then be lost. A certain minimum thickness is also required. Too thin a crust can be peeled off by a strong wind. It is normally sufficient to bind the top inch or so to produce a cohesive cover that will prevent material loss. In reality, the minimum effective thickness will depend on both the bulk material and the environment in which the crust is expected to survive; however, in general, thicker crusts are preferred.
Of particular note to this disclosure is Callahan's (U.S. Pat. No. 4,369,121) teaching of the use of water-soluble cellulose ethers with an additional wetting agent as a dust palliative. Water-soluble cellulose ethers are well-known in the coatings industry. Their behavior with plasticizers has been studied extensively. As long ago as 1940 it was recognized that the addition of a plasticizer weakened the tensile strength of cellulose ether films (Kropscott, U.S. Pat. No. 2,226,823). Oakley made a similar observation (Oakley, U.S. Pat. No. 2,653,108). More recently Part, et. al. quantified the same behavior specifically in methyl cellulose and hydroxypropyl methyl cellulose using, among other plasticizers, propylene glycol and glycerin (2,3-hydroxy-1-propanol) (Park, H. J., Weller, C. L., Vergano, P. J., and Testin, R. F.; Journal of Food Science, 58, #6, 1993 pp 1361-1364). A Dow Chemical website discusses not only strength, but film toughness and Young's Modulus (http://dowwolff.custhelp.com/app/answers/detail/a_id/2357/˜/methocel-effect-of-plasticizers-on-film-properties-in-tablet-coatings). In every case the addition of glycerin or propylene glycol weakened the films. Marsden (U.S. Published Patent Application No. 20100301266) similarly cautions that plasticizers in his guar gum formulations “ . . . cannot unduly decrease the strength of the matrix.” It is therefore a surprising and unexpected result that the addition of a plasticizer to coatings similar to those described by Callahan would result in improved strength.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior dust suppression fluids of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.